1. Field of the Invention
The present disclosure relates to semiconductor lasers and, more particularly, to an optical waveguide structure that is resistant to misfit defect formation and achieves relatively high optical confinement in the active region of the waveguide structure. Semiconductor lasers according to the present disclosure are particularly well suited, for example, as electrically pumped green laser diodes.
2. Technical Background
The present inventors have recognized the advantage of using semipolar substrates in the design and fabrication of electrically pumped green laser diodes based on group-III nitride compounds because of potentially higher optical gain and uniformity, particularly in the case of active regions made of InGaN quantum wells. The present inventors have also recognized that misfit defect formation is a particularly acute problem in the context of laser diode structures grown on semipolar substrates because, during the growth of AlGaN or InGaN layers, strong tensile and compressive strain accumulate and lead to the formation of misfit dislocations upon strain relaxation. These misfit dislocations can lead to poor emission efficiency and reliability.
The present inventors have recognized that the presence of Al in the waveguide layer of a laser diode is generally considered to be undesirable because AlGaN and AlInGaN are believed to have lower refractive index compared to similar materials without Al, such as GaN and InGaN. So, the general trend is to keep Al away from the waveguide core as much as possible. Nevertheless, the present inventors have recognized that the use of Al and InGaN in the waveguide layers of a semiconductor laser diode can be advantageous because the Al-containing layers provide strain compensation for InGaN layers, but do not hinder the high refractive index provided by the InGaN layers. This enhances optical confinement and optical gain while keeping high structure crystalline quality.